issue_templates: add Release_EOL.md

Move the contents of this wiki page into an issue template and modernize
it a bit while at it:
https://wiki.postmarketos.org/wiki/Creating_a_release_branch#Drop_phase

Signed-off-by: Oliver Smith <ollieparanoid@postmarketos.org>
Part-of: <https://gitlab.postmarketos.org/postmarketOS/pmaports/-/merge_requests/8486>

.gitlab/issue_templates/Release_EOL.md

@@ -0,0 +1,29 @@
+<!--
+    This template is for marking a postmarketOS stable release as EOL.
+
+    Name this issue "vYY.MM EOL".
+-->
+
+When the release has reached its EOL date:
+
+- [ ] `postmarketos-base-ui`: let it depend on
+  `postmarketos-release-upgrade-notification` and bump pkgrel
+  - [ ] ensure it builds locally with pmbootstrap
+  - [ ] push it to the branch
+  - [ ] make sure bpo built and released the updated packages
+- [ ] drop the old release from
+  [monitoring](https://gitlab.postmarketos.org/postmarketOS/monitoring)
+- [ ] drop the "oldold -> old" release check from
+  [postmarketos-release-upgrade](https://gitlab.postmarketos.org/postmarketOS/postmarketos-release-upgrade/)
+  CI
+- [ ] bpo: configure images so they don't get built for the old release anymore
+- [ ] bpo: configure branches so we don't try to build packages for old
+  branches anymore
+  - e.g. on startup all branches are checked for new packages, we should keep
+    the amount of branches that get checked minimal
+- [ ] pmaports.git channels.cfg: change description:
+  - "Old release (unsupported)"
+- [ ] Update the [Releases](https://docs.postmarketos.org/pmaports/main/releases.html) page
+  - Move the release from active to old
+- [ ] Consider removing images for the previous releases to save disk space
+  (people can build their own images with pmbootstrap if needed)